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# MethodHandleInfo

`public interface MethodHandleInfo`
``

java.lang.invoke.MethodHandleInfo

A symbolic reference obtained by cracking a direct method handle into its consitutent symbolic parts. To crack a direct method handle, call `Lookup#revealDirect`.

# Direct Method Handles

A direct method handle represents a method, constructor, or field without any intervening argument bindings or other transformations. The method, constructor, or field referred to by a direct method handle is called its underlying member. Direct method handles may be obtained in any of these ways:

- By executing an `ldc` instruction on a `CONSTANT_MethodHandle` constant. (See the Java Virtual Machine Specification, sections 4.4.8 and 5.4.3.)
- By calling one of the Lookup Factory Methods, such as `Lookup#findVirtual`, to resolve a symbolic reference into a method handle. A symbolic reference consists of a class, name string, and type.
- By calling the factory method `Lookup#unreflect` or `Lookup#unreflectSpecial` to convert a `Method` into a method handle.
- By calling the factory method `Lookup#unreflectConstructor` to convert a `Constructor` into a method handle.
- By calling the factory method `Lookup#unreflectGetter` or `Lookup#unreflectSetter` to convert a `Field` into a method handle.

# Restrictions on Cracking

Given a suitable `Lookup` object, it is possible to crack any direct method handle to recover a symbolic reference for the underlying method, constructor, or field. Cracking must be done via a `Lookup` object equivalent to that which created the target method handle, or which has enough access permissions to recreate an equivalent method handle.

If the underlying method is caller sensitive, the direct method handle will have been "bound" to a particular caller class, the lookup class of the lookup object used to create it. Cracking this method handle with a different lookup class will fail even if the underlying method is public (like `Class.forName`).

The requirement of lookup object matching provides a "fast fail" behavior for programs which may otherwise trust erroneous revelation of a method handle with symbolic information (or caller binding) from an unexpected scope. Use `MethodHandles.reflectAs(Class, MethodHandle)` to override this limitation.

# Reference kinds

The Lookup Factory Methods correspond to all major use cases for methods, constructors, and fields. These use cases may be distinguished using small integers as follows:

reference kind | descriptive name | scope | member | behavior
--- | --- | --- | --- | ---
`1` | `REF_getField` | `class` | `FT f;` | `(T) this.f;`
`2` | `REF_getStatic` | `class` or `interface` | `static` `FT f;` | `(T) C.f;`
`3` | `REF_putField` | `class` | `FT f;` | `this.f = x;`
`4` | `REF_putStatic` | `class` | `static` `FT f;` | `C.f = arg;`
`5` | `REF_invokeVirtual` | `class` | `T m(A*);` | `(T) this.m(arg*);`
`6` | `REF_invokeStatic` | `class` or `interface` | `static` `T m(A*);` | `(T) C.m(arg*);`
`7` | `REF_invokeSpecial` | `class` or `interface` | `T m(A*);` | `(T) super.m(arg*);`
`8` | `REF_newInvokeSpecial` | `class` | `C(A*);` | `new C(arg*);`
`9` | `REF_invokeInterface` | `interface` | `T m(A*);` | `(T) this.m(arg*);`

## Summary

### Constants
`int` | `REF_getField` A direct method handle reference kind, as defined in the table above.
`int` | `REF_getStatic`
`int` | `REF_invokeInterface`
`int` | `REF_invokeSpecial`
`int` | `REF_invokeStatic`
`int` | `REF_invokeVirtual`
`int` | `REF_newInvokeSpecial`
`int` | `REF_putField`
`int` | `REF_putStatic`

### Public methods
`abstract Class<?>` | `getDeclaringClass()` Returns the class in which the cracked method handle's underlying member was defined.
`abstract MethodType` | `getMethodType()` Returns the nominal type of the cracked symbolic reference, expressed as a method type.
`abstract int` | `getModifiers()` Returns the access modifiers of the underlying member.
`abstract String` | `getName()` Returns the name of the cracked method handle's underlying member.
`abstract int` | `getReferenceKind()` Returns the reference kind of the cracked method handle, which in turn determines whether the method handle's underlying member was a constructor, method, or field.
`default boolean` | `isVarArgs()` Determines if the underlying member was a variable arity method or constructor.
`static boolean` | `refKindIsField(int refKind)` This method is deprecated. This internal method was accidentally added to API 26 and must not be used. No replacement is available but it is possible to replicate using information from the table above, e.g. `refKind >= 1 && refKind <= 4`. There are no guarantees that this logic will work if future versions extend the table.
`static boolean` | `refKindIsValid(int refKind)` This method is deprecated. This internal method was accidentally added to API 26 and must not be used. No replacement is available but it is possible to replicate using information from the table above, e.g. `refKind >= 1 && refKind <= 9`. There are no guarantees that this logic will work if future versions extend the table.
`static String` | `refKindName(int refKind)` This method is deprecated. This internal method was accidentally added to API 26 and must not be used. Use `MethodHandleInfo#referenceKindToString(int)` instead.
`static String` | `referenceKindToString(int referenceKind)` Returns the descriptive name of the given reference kind, as defined in the table above.
`abstract <T extends Member> T` | `reflectAs(Class<T> expected, MethodHandles.Lookup lookup)` Reflects the underlying member as a method, constructor, or field object.
`static String` | `toString(int kind, Class<?> defc, String name, MethodType type)` Returns a string representation for a `MethodHandleInfo`, given the four parts of its symbolic reference.

## Constants

### REF_getField

    public static final int REF_getField

A direct method handle reference kind, as defined in the table above.

Constant Value: 1 (0x00000001)

### REF_getStatic

    public static final int REF_getStatic

Constant Value: 2 (0x00000002)

### REF_invokeInterface

    public static final int REF_invokeInterface

Constant Value: 9 (0x00000009)

### REF_invokeSpecial

    public static final int REF_invokeSpecial

Constant Value: 7 (0x00000007)

### REF_invokeStatic

    public static final int REF_invokeStatic

Constant Value: 6 (0x00000006)

### REF_invokeVirtual

    public static final int REF_invokeVirtual

Constant Value: 5 (0x00000005)

### REF_newInvokeSpecial

    public static final int REF_newInvokeSpecial

Constant Value: 8 (0x00000008)

### REF_putField

    public static final int REF_putField

Constant Value: 3 (0x00000003)

### REF_putStatic

    public static final int REF_putStatic

Constant Value: 4 (0x00000004)

## Public methods

### getDeclaringClass

    public abstract Class<?> getDeclaringClass ()

Returns the class in which the cracked method handle's underlying member was defined.

Returns
`Class<?>` | the declaring class of the underlying member

### getMethodType

    public abstract MethodType getMethodType ()

Returns the nominal type of the cracked symbolic reference, expressed as a method type. If the reference is to a constructor, the return type will be `void`. If it is to a non-static method, the method type will not mention the `this` parameter. If it is to a field and the requested access is to read the field, the method type will have no parameters and return the field type. If it is to a field and the requested access is to write the field, the method type will have one parameter of the field type and return `void`.

Note that original direct method handle may include a leading `this` parameter, or (in the case of a constructor) will replace the `void` return type with the constructed class. The nominal type does not include any `this` parameter, and (in the case of a constructor) will return `void`.

Returns
`MethodType` | the type of the underlying member, expressed as a method type

### getModifiers

    public abstract int getModifiers ()

Returns the access modifiers of the underlying member.

Returns
`int` | the Java language modifiers for underlying member, or -1 if the member cannot be accessed

See also:

- `Modifier`
- `reflectAs(Class, MethodHandles.Lookup)`

### getName

    public abstract String getName ()

Returns the name of the cracked method handle's underlying member. This is `"<init>"` if the underlying member was a constructor, else it is a simple method name or field name.

Returns
`String` | the simple name of the underlying member

### getReferenceKind

    public abstract int getReferenceKind ()

Returns the reference kind of the cracked method handle, which in turn determines whether the method handle's underlying member was a constructor, method, or field. See the table above for definitions.

Returns
`int` | the integer code for the kind of reference used to access the underlying member

### isVarArgs

    public boolean isVarArgs ()

Determines if the underlying member was a variable arity method or constructor. Such members are represented by method handles that are varargs collectors.

##### Implementation Requirements:

- This produces a result equivalent to:

      getReferenceKind() >= REF_invokeVirtual && Modifier.isTransient(getModifiers())

Returns
`boolean` | `true` if and only if the underlying member was declared with variable arity.

### refKindIsField

    public static boolean refKindIsField (int refKind)

This method is deprecated.
This internal method was accidentally added to API 26 and must not be used. No replacement is available but it is possible to replicate using information from the table above, e.g. `refKind >= 1 && refKind <= 4`. There are no guarantees that this logic will work if future versions extend the table.

Parameters
`refKind` | `int`

Returns
`boolean` |

### refKindIsValid

    public static boolean refKindIsValid (int refKind)

This method is deprecated.
This internal method was accidentally added to API 26 and must not be used. No replacement is available but it is possible to replicate using information from the table above, e.g. `refKind >= 1 && refKind <= 9`. There are no guarantees that this logic will work if future versions extend the table.

Parameters
`refKind` | `int`

Returns
`boolean` |

### refKindName

    public static String refKindName (int refKind)

This method is deprecated.
This internal method was accidentally added to API 26 and must not be used. Use `MethodHandleInfo#referenceKindToString(int)` instead.

Parameters
`refKind` | `int`

Returns
`String` |

### referenceKindToString

    public static String referenceKindToString (int referenceKind)

Returns the descriptive name of the given reference kind, as defined in the table above. The conventional prefix "REF_" is omitted.

Parameters
`referenceKind` | `int`: an integer code for a kind of reference used to access a class member

Returns
`String` | a mixed-case string such as `"getField"`

Throws
`IllegalArgumentException` | if the argument is not a valid reference kind number

### reflectAs

    public abstract T reflectAs (Class<T> expected,
                    MethodHandles.Lookup lookup)

Reflects the underlying member as a method, constructor, or field object. If the underlying member is public, it is reflected as if by `getMethod`, `getConstructor`, or `getField`. Otherwise, it is reflected as if by `getDeclaredMethod`, `getDeclaredConstructor`, or `getDeclaredField`. The underlying member must be accessible to the given lookup object.

Parameters
`expected` | `Class`: a class object representing the desired result type `T`
`lookup` | `MethodHandles.Lookup`: the lookup object that created this MethodHandleInfo, or one with equivalent access privileges

Returns
`T` | a reference to the method, constructor, or field object

Throws
`ClassCastException` | if the member is not of the expected type
`NullPointerException` | if either argument is `null`
`IllegalArgumentException` | if the underlying member is not accessible to the given lookup object

### toString

    public static String toString (int kind,
                    Class<?> defc,
                    String name,
                    MethodType type)

Returns a string representation for a `MethodHandleInfo`, given the four parts of its symbolic reference. This is defined to be of the form `"RK C.N:MT"`, where `RK` is the reference kind string for `kind`, `C` is the name of `defc` `N` is the `name`, and `MT` is the `type`. These four values may be obtained from the reference kind, declaring class, member name, and method type of a `MethodHandleInfo` object.

##### Implementation Requirements:

- This produces a result equivalent to:

      String.format("%s %s.%s:%s", referenceKindToString(kind), defc.getName(), name, type)

Parameters
`kind` | `int`: the reference kind part of the symbolic reference
`defc` | `Class`: the declaring class part of the symbolic reference
`name` | `String`: the member name part of the symbolic reference
`type` | `MethodType`: the method type part of the symbolic reference

Returns
`String` | a string of the form `"RK C.N:MT"`

Throws
`IllegalArgumentException` | if the first argument is not a valid reference kind number
`NullPointerException` | if any reference argument is `null`

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